Having to explain your colleagues or friends the content you add in your spreadsheet, is like doing double the work. Formulas, graphs and other cells can save much time if they are accompanied by a short, yet effective comment. Inserting comments can prove to be useful reminders and can save ample of time from cross-referencing other workbooks.
A red mark in the corner of a cell will indicate that a note is attached. Resting the pointer of your mouse on that very cell will enable the comment to pop up.
The shortcut for creating a comment for Windows users is:
⇧+Fn+F2 - Inserting or editing a comment
⇧+Fn+ F10 - Deleting a comment
You have learned the shortcut to insert new comments, now know the Excel Shortcut To Insert New Sheet and Excel Shortcut For New Workbook